About Biodegradability

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Home compost

Nature already has ways of recycling its polymers and the same process will also break down some of our synthetic ones. There is a big difference between home composting and industrial composting (they are populated by different microbial species) and unfortunately the two are easily confused.

Biodegradation in Terrestrial Environments

The fate of biodegradable plastics depends on their disposal environment. Some materials are quite broadly biodegradable, whereas others only degrade in very specific conditions.

Unlike food products, bioplastics are not currently required to disclose their ingredients. Products simply state which environments they can biodegrade in.

Compost

Composting can be an option for recycling certain waste plastics that would otherwise go to conventional landfill. It is particularly suited to items that are irretrievably mixed with green agricultural waste (such as clips, labels, pots, etc.), or for plastics too contaminated to be suitable for conventional recycling. However, it is not a solution for general plastic disposal.more

When it is employed, industrial composting is the fastest way to biodegrade plastics. The heaps are carefully managed, with temperature, moisture content, and the mix of organic waste all being suitably controlled. Under these optimal conditions, pretty much all biodegradable plastics will decompose fairly rapidly, including PLA.more

(TUV Austria lists literally hundreds of bioplastic compounds that are certified for industrial composting. Many of these are probably PLA based, because it's cheap and easy to work with.)

The drawback is that the industrial composting process lasts only a few months, and only films and very thin-walled items stand a chance of fully biodegrading by then. Any undegraded fragments that remain in the finished compost will likely end up being dug into the soil, and this is a problem because not all compostable plastics will continue to biodegrade in nature.

As discussed on the previous page, home composting conditions are considerably less effective, and the introduction of plastic waste is not a good idea. Even products labelled as suitable for home composting cannot be relied upon and may simply result in contamination.

Soil

A plastic tree guard discarded into nature

Plastic tree guards & shelters are widely used to protect young saplings from animals. But they can end up going astray, along with the (usually) nylon cable ties used to fit them. Since many will never be recovered, biodegradable plastics make good sense in these situations.

Soil biodegradable plastics are potentially more beneficial because here biodegradability is an intrinsic material property. Decomposition is therefore a matter of when, not if.more

Products made from these particular materials can be helpful in reducing long term environmental pollution, and they are also safer for disposal via composting. (Because any remaining fragments will continue to degrade even if they do end up in soil.)

Although soil is not an intentional disposal environment, plastic products do frequently escape into the wild. In applications where this can occur, the use of soil biodegradable plastics makes good sense.

Unfortunately, the choice is very limited, with TUV Austria listing only about 20 different resin suppliers at the end of 2023.

Most of these TUV certified compounds are bacterial polyesters (PHAs) or polysaccharide derivatives (starch & cellulose), all of which are naturally sourced and therefore highly biodegradable in most environments. However, they also tend to be weak and difficult to process. Although they have been around for many decades (and are consequently well-studied), these material are not widely used for moulded items, and are better suited for making bags & films.

The biodegradable thermoplastics PBAT and PBS are relatively new alternatives. Being artificial they do not degrade as rapidly, but they can be moulded just like conventional plastics and have become increasingly popular since their arrival in the late 1990s.more Radio-carbon labelling has been used to confirm the soil biodegradability of PBAT,more and our bioplastic i-Ties likely contain this polymer.more

A much older alternative for soil biodegradation is PCL (polycaprolactone) which has been around since the 1930s. Despite being very biodegradable (it is also certified for marine applications), PCL has never been very widely used, mostly because it is quite weak and fairly expensive.more Using it for engineering purposes (like making cable ties) is very challenging but possible.

Many blends of these polymers have also been developed to extend their useful properties, but they still do not provide the same versatility as conventional thermoplastics. Combined with the added cost of producing them, soil biodegradable plastic products remain quite niche, but they can be relevant for outdoor applications where pollution is a concern.

Landfill

Landfill sites can be highly variable, with some areas being very bioactive and others essentially inert. It is not uncommon to find both extremes just metres aparts.

Although not quite dry, sealed tombs, they are typically low moisture and largely anaerobic environments. Biodegradation is very slow under such conditions and organic material can be suprisingly durable. (Decades old newspapers are commonly found intact in many landfill sites, and it has been reported that even food waste can take over 20 years to decompose.)

Unsurprisingly, even the best biodegradable plastics will do no better than this, so anything that goes into landfill is essentially being parked rather than returned to nature.

A newer generation of landfills act as bioreactors, deliberately encouraging decomposition to generate methane biogas for energy. Numbers are currently low and the fate of biodegradable plastics has not been extensively studied just yet, but it is likely that most would break down relatively quickly in these types of landfill.more

rapstrap on coral
Biodegradable plastics for marine applications are scarce, especially those suitable for engineering complex items. Rapstraps at least offer a waste-free design that can be made using renewable-resource polymers.

Aquatic Conditions

Much plastic litter ends up in marine and freshwater environments, and there is no easy solution since very few bioplastics can decompose in water.

PVOH (or PVA) can, and it is generally considered OK for disposal in water. Although more commonly used as an adhesive, it is possible to mould it or convert it into film, which makes it a fairly versatile polymer. Waste-water treatment plants harbour diverse bacterial species that routinely feed on the dissolved detergent pouches that get flushed away every day.more Studies have also identified numerous fungal species that can degrade PVOH in soil. more

On the downside, PVOH products cannot be used in wet conditions, and are therefore not really suitable for outdoor applications. Once exposed to water, thin items will quickly dissolve, and larger items (such as our PVOH i-Ties) will go sticky and swell up before dissolving.more

Another option are the di-alcohol polyesters (such as PBAT and PBS) which may be able to (slowly) degrade in some conditions. Research is still quite limited, and in a recent Brazilian experiment, no significant biodegradation of pure PBAT test plaques was seen after suspension in seawater for 6 months.more However, slightly better results have been reported for freshwater sediment burial of PBAT film.more Whether these (or similar) polymers will prove useful in reducing aquatic plastic pollution is still uncertain.

microbial growth on a biodegradable starch-polyester rapstrap

Bio-film growing on a BIO23 i-Tie, which probably contains PBAT or PBS. This sample was submerged for over a year in fresh water at ambient temperature. Biodegradation is extremely slow under these dilute anoxic conditions, and the underlying polymer may not be degrading very much at all.

By constrast, the hydroxy-acid polyesters do generally biodegrade quite well in both marine and freshwater conditions. This group includes the bacterial PHAs (which are naturally produced and fully biodegradable), and also the glycolic acid polymer PGA, which happens to hydrolyse rapidly in water even at ambient temperatures. However, PHAs can be difficult to melt-process, and PGA is quite stiff. (Neither is particularly suitable for moulding cable ties.)

PCL (poly-caprolactone) is another hydroxy-acid polyester, and this currently seems to be the best polymer for aquatic applications. PCL happens to be similar to the natural plant polyesters cutin & suberin, and is known to be biodegradable in most ambient environments, including the oceans.more

PCL has similar physical properties to polyethylene and is relatively easy to work with. It is also quite flexible and modestly strong, but unfortunately suffers from a low yield point; So far we have not been able to produce a viable rapstrap with it.more

Interestingly, the lactic acid polymer PLA is also a hydroxy-acid polyester, yet it is not very biodegradable in the environment. There is little evidence that PLA readily biodegrades in soil, and there is essentially no evidence that it biodegrades in marine or freshwater conditions.more Few microbial species seem able to decompose PLA, and those that can generally require hot conditions.more

Other melt-processable polymers worthy of note are some of the polysaccharides, particularly thermoplastic starch and cellulose acetate. TPS is cheap, but also quite weak and dimensionally unstable (it is hygroscopic and readily absorbs moisture). It is therefore usually blended with other polymers as a compromise.

CA is quite the opposite, being fairly strong and rigid, but also more expensive. There has been some interesting research from WHOI over the last few years demonstrating that cellulose diacetate degrades quite rapidily in the oceans, especially if turned into a foam.more

Other Concerns

Shallow freshwater stream

Shallow streams can be quite biodiverse, and are not the worst location for biodegradable plastics to end up. Certainly for polyesters, the ongoing presence of water is a major factor in their biodegradation.

How biodegradable a plastic product will be in fresh or salt water conditions depends greatly on where it ends up: There is a big difference between river, coastal and deep ocean locations since temperature, pressure, UV-exposure, oxygen levels, mechanical weathering and microbial diversity are all quite different (amongst many other things).more

Most biodegradable polymers are denser than water, and in calm conditions, solid items will tend to sink into the bottom sediment. In shallow coastal waters (and in rivers & lakes), sediment can be a relatively nutrient rich environment compared to the more dilute water body itself. For polymers amenable to anaerobic biodegradation, sedimentary conditions can be quite effective.more

Plastic films, however, are more likely to float, especially if intermixed with polyolefin litter. (Polypropylene & polyethylene are both less dense than water and can buoy up other materials.) Surface conditions present a totally different environment, but films do at least provide a large surface area which can accelerate biodegradation.

Although much research has been published on aquatic biodegradation, long term experiments (especially in deep ocean conditions) are lacking since they are more difficult to conduct than on land. Simulated laboratory tests are also not a good proxy for real-world conditions.

Anaerobic Digestors

Anaerobic digestors take agri-food waste and ferment it into biogas, typically at elevated temperatures. (Above 30°C, sometimes much hotter). The residues are then usually sterilised and used as crop fertilizer.more However, plastics are frequently present in the feedstock, and these contaminate the output.more

This is being mitigated in some countries by the broader use of biodegradable polymers, which in theory will break down during fermentation. However, some contamination still remains: A recent report from Germany found PLA and PBAT microplastics (probably from bags) in both liquid and solid AD waste. The PLA is of particular concern since it is known to be quite persistent in the ambient environment.more

The underlying problem is twofold. The digestion rates of even the best biodegradable polymers are quite slow compared to natural organic wastes; And AD operators want to run their plants as fast as possible. As a consequence, the digestion process does not usually continue for long enough to fully break down bioplastics.

anaerobic digestion of cellulose and polycaprolactone

Anaerobic digestion is mostly performed by bacteria. They can degrade cellulose by 80% in about a week, but it takes over 2 months to reach the same level with PCL. (Data from Ingevity, tests performed at Fraunhofer UMSICHT.)

This chart from Ingevity shows that their high molecular weight PCL takes 8x longer to digest than cellulose. This is a little surprising because polycaprolactone is one of the most biodegradable thermoplastic polymers currently available, and it takes only about 2x longer in aerobic soil & composting conditions.

Given this very slow rate of decomposition, even operating anaerobic digesters for longer cycles will not fully solve the problem. The output quality could, however, be improved by the wider use of soil-degradable biopolymers. This would at least help ensure that any residual plastics would continue to decompose once they entered the natural environment.

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